Variable conductance heat pipe

ABSTRACT

A variable conductance heat pipe is provided. The variable conductance heat pipe includes a sealed container in which a working fluid and a noncondensable gas are sealed. The sealed container includes a heat receiving portion to which an element to be cooled is provided, and heat radiating portion. An amount of heat is supplied to the heat receiving portion when the element to be cooled is in a waiting state.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2008-219550, filed on Aug. 28, 2008, the entire subject matter of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable conductance heat pipe for maintaining the temperature of equipment not in operation at an arbitrary temperature with small electric power.

2. Description of the Related Art

In a projector and a printer, in order to operate desirably, the temperature of a main part such as a light source has to be controlled to an appropriate temperature. As a temperature controller for maintaining the temperature of such a main part of a projector or printer to a temperature suitable for a desired function, there has been proposed a heater-type temperature controller for implementing a heat control through heating by a heater, a heat pump-type temperature controller for implementing a heat control through heating by a heater or through heating and cooling by a heat pump, and a Peltier device-type temperature controller for implementing a temperature control through current control or current reversal by making use of the Peltier effect. When a projector or printer becomes in operation, if a temperature of the main part of the projector or printer in a waiting state (not in operation) and a set temperature at which the main part operates are different, a time for temperature control (a waiting time) is required so that the temperature of the main part which is not in operation is controlled to the set temperature by the temperature controller. The larger the difference between the two temperatures is, the longer the waiting time becomes, which is not convenient for the user. Accordingly, there have been proposed a projector and a printer in which a temperature controller is maintained in operation to perform the temperature control in advance even when the projector or printer is not in operation, so as to reduce the waiting time when the projector or printer is operated.

Further, an image forming apparatus detects a time period during which a state continues in which no request is received from the user, so as to switch operation modes and change set heating temperatures to thereby realize the conservation of electric power and enable a quick output in response to a request from the user (for example, see JP-A-2005-49621 (page 13, FIG. 1)).

In a system having electronic equipment whose temperature needs to be controlled, although the temperature of the electronic equipment is desirably maintained in a state where the temperature lies in the vicinity of a set temperature at which the electronic equipment operates at all times in order to enable a quick output in response to a request from the user, when the temperature of the electronic equipment is maintained in such a state at all times, the consumed power that is necessary to do so becomes large, which results in a wasteful use of electric power. Then, in the image forming unit described in JP-A-2005-49621, there is provided a device configured to detect a time period during which a state continues in which there is no request from the user, so as to switch operation modes and reduce the set temperature at which the electronic equipment is maintained heated, whereby the consumed power of the electronic equipment while it is not in operation is attempted to be reduced. In this approach, however, since equipment for detecting an input from the user and a control mechanism for enabling the detection are required, electric power for controlling them is required separately, and since the electronic equipment is heat controlled (heated) to the set temperature while heat is radiated from a heat radiating unit of the electronic equipment, the reduction effect of consumed power is not as high as expected.

In addition, although there is an approach in which a temperature control speed at which the temperature of electronic equipment is controlled to its set temperature is made faster by employing high-performance heating and cooling units, a high-level control with a short response time becomes necessary, and such a control is difficult to be applied to electronic equipment which is small in size.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a variable conductance heat pipe comprising a sealed container in which a working fluid and a noncondensable gas are sealed, the sealed container including a heat receiving portion to which an element to be cooled is provided, and heat radiating portion. An amount of heat is supplied to the heat receiving portion when the element to be cooled is in a waiting state.

According to the above-described configuration, the variable conductance heat pipe can operate as a heat insulated type heat pipe with less electric power by heat being supplied to the heat receiving portion, the temperature of the heat receiving portion can be set to an arbitrary temperature based on the amount of noncondensable gas sealed in the variable conductance heat pipe, and the temperature of the element to be cooled provided in the heat receiving portion can easily be controlled to an arbitrary temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in detail based on the following figures, wherein:

FIG. 1 is a diagram showing the configuration of a variable conductance heat pipe according to Embodiment 1 of the invention;

FIG. 2 is a chart showing the heat load depending property of temperature for the variable conductance heat pipe and a normal heat pipe;

FIG. 3 is a diagram showing the configuration of a variable conductance heat pipe according to Embodiment 2 of the invention;

FIG. 4 is a diagram showing the configuration of a variable conductance heat pipe according to Embodiment 3 of the invention; and

FIG. 5 is a diagram showing the configuration of a variable conductance heat pipe according to Embodiment 4 of the invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

FIG. 1 is a sectional view showing a variable conductance heat pipe according to Embodiment 1 of the invention. As shown in FIG. 1, from one end portion, a variable conductance heat pipe 20 includes a sealed container 1 are a heat receiving portion 2 (an evaporating portion), a heat insulating portion 3 (a transporting portion), a heat radiating portion 4 (a condensing portion) and a noncondensable gas reservoir portion 5. A working fluid (a liquid 6 and vapor 7 thereof) and a noncondensable gas 8 are sealed in an interior of the sealed container 1. A heater 40 and an element to be cooled 9 are provided to the heat receiving portion 2.

Next, the operation of the variable conductance heat pipe of the Embodiment 1 will be described. When the element to be cooled 9 is activated to operate to obtain a desired function of the element to be cooled 9, heat is generated in an interior of the element to be cooled 9, whereby the temperature of the element to be cooled 9 is raised. The heat receiving portion 2 contacts (is connected to) the element to be cooled 9 and the heat radiating portion 4 contacts (is connected to) a heat sink 10, whereby heat is conducted from the element to be cooled 9 whose temperature is higher to the heat receiving portion 2. The heat is then conducted further to the liquid 6 residing within the heat receiving portion 2, and the heat conducted to the liquid 6 is absorbed by the liquid 6 in the form of latent heat or the liquid 6 is evaporated or boiled, whereby vapor 7 is generated. The vapor 7 or the vapor 7 and the liquid 6 flow into the heat radiating portion 4 via the heat insulating portion 3, where latent heat possessed by the vapor 7 is emitted to the heat radiating portion 4 while the vapor 7 is condensed, the heat so emitted being radiated to the heat sink 10 whose temperature is lower than that of the heat radiating portion 4. As this occurs, the condensed liquid (the liquid 6) which is generated by the vapor 7 being condensed is returned to the heat receiving portion 2 from the heat radiating portion 4 via the heat insulating portion 3 by virtue of gravity or capillary force. Heat generated in the element to be cooled 9 is transmitted (discharged) continuously to the heat sink 10 by circulation of these vapor 7 and liquid 6.

On the other hand, the noncondensable gas 8 which is sealed within the interior of the sealed container 1 is caused to move to the noncondensable gas reservoir portion 5 or a portion of the heat radiating portion 4 which lies to face the noncondensable gas reservoir portion 5 via the heat insulating portion 3 and the heat radiating portion 4 as the vapor 7 or the vapor 7 and liquid 6 move and is accumulated to be retained therein. When the noncondensable gas 8 stays as described above, the vapor 7 is made difficult to enter the noncondensable gas 8, thereby an interface 11 being formed between the vapor 7 and the noncondensable gas 8. The vapor 7 pushes the noncondensable gas 8 continuously, whereby the interface 11 is caused to move, and the pressures of the vapor 7 and the noncondensable gas 8 reach an equilibrium state, whereupon the interface 11 stops moving, and its position is stabilized. Consequently, (1) when the interface 11 reaches to be positioned in the noncondensable gas reservoir portion 5, since the vapor 7 is condensed over the whole heat radiating portion 4, a heat radiating capability of 100% can be obtained, (2) when the interface 11 is positioned within the heat radiating portion 4, since the area over which the vapor 7 is condensed (the heat radiating area) is reduced, the heat radiating capability decreases (the heat radiating capability is variable in such a manner that 0%<heat radiating capability<100%). In addition, (3) when the interface 11 is positioned within the heat insulating portion 3 or the heat receiving portion 2, heat can be insulated (0% of the heat radiating capability can be obtained). However, since part of heat is caused to move by conduction of heat through the wall of the sealed container 1, in reality, although it is not large, there exists the heat radiating capability.

What has been described above is an operation principle of the variable conductance heat pipe 20, and the temperature of the element to be cooled 9 is controlled through expansion and contraction of the noncondensable gas 8 by the element to be cooled 9 being activated to operate and is allowed to be maintained at an arbitrary set temperature. However, when the element to be cooled 9 stops operating, the generation of vapor 7 is stopped, and the temperatures of the noncondensable gas reservoir portion 5, the heat radiating portion 4, the heat insulating portion 3, the heat receiving portion 2 and the element to be cooled 9 are reduced to a temperature equal to that of the heat sink 10. When the element to be cooled 9 is activated to operate again in this state, a time delay (a waiting time) occurs before the temperature of the element to be cooled 9 is raised to the set temperature. Then, in this embodiment, an amount of heat which enables the interface 11 to be positioned to realize the states described under (2) and (3) above is supplied by the heater 40 which is provided to the heat receiving portion 2, so that the variable conductance heat pipe is maintained operating in such a state that the heat radiating portion 4 is generally heat insulated, whereby the temperature of the heat receiving portion 2 is made to be maintained at an arbitrary set temperature with a small amount of heat. In addition, the heater 40 may be activated to operate irrespective of the element to be cooled 9 being in operation or not, or the heater 40 may be operated cooperatively with the element to be cooled 9 in such a manner that the heater 40 is activated to operate when the element to be cooled 9 is not in operation (for example, a power supply for the element to be cooled 9 and a power supply for the heater 40 are switched over). In addition, the heater 40 may be activated to operate by detecting that the element to be cooled 9 is not in operation (the heater 40 is switched on by detecting electrically that the operation of the element to be cooled 9 is switched off or by detecting it by a temperature sensor).

FIG. 2 shows an example of heat load depending property of the temperature of the heat receiving portion 2 of the variable conductance heat pipe 20 or how the temperature of the heat receiving portion 2 of the variable conductance heat pipe 20 is dependent on heat load. In the figure, a ♦ mark denotes a normal heat pipe (which employs no noncondensable gas) in which the temperature of a heat receiving portion increases linearly as heat load increases (this is also true in a normal cooling apparatus), and a ▪ mark denotes the variable conductance heat pipe according to the Embodiment 1, in which the temperature of the heat receiving portion reaches generally 50° C. which is close to the set temperature (the temperature at which the element to be cooled 9 operates, for example, 53° C.) by a heat load of the order of 5 W being applied thereto. Namely, when the element to be cooled 9 is waiting for activation while it is not in operation or when the element to be cooled 9 is in a waiting state, if a heat amount of 5 W is made to be generated in the heater 40, the element to be cooled 9 can be heated to 50° C. while it is in the waiting state. Even if the element to be cooled 9 is activated to operate in this state and the heat value when it is in operation reaches, for example, 50 W, since the temperature of the element to be cooled 9 only has to be raised by 3° C. to reach 53° C., a time period required for the element to be cooled 9 to reach its steady state is short. In addition, in most electric apparatuses, since the element to be cooled 9 can operate almost equally in a temperature range of 50° C. to 53° C., the operation of the element to be cooled 9 becomes stable immediately after it is activated to operate. As is seen from FIG. 2, the normal heat pipe requires about 65 W for the element to be cooled to be set to 53° C., while the variable conductance heat pipe according to the Embodiment 1 only requires a heat load which is one thirteenth the heat load of the normal heat pipe before the temperature which is extremely close to the set temperature can be reached. When the normal heat pipe is used for cooling, since a difference of as much as 33 k is generated between the set temperature and the ambient temperature, when the heat control is carried out through heating by the heater, the heat load of 65 W has to be inputted to the element to be cooled while it is in the waiting state. Because of this, a large amount of energy has to be wasted, and in order to make the input of such a large heat load happen, a heater of a large capacity, as well as wiring and power supply for a large electric current have to be provided, which is difficult to be realized on an electric apparatus which is small in size.

As has been described above, according to Embodiment 1, the time period required for the element to be cooled to reach from the waiting state to the steady state, that is, the time period required for the element to be cooled to obtain its desired function is short, and the ensured temperature control can be implemented. In addition, since almost a heat insulating state is obtained while the element to be cooled 9 is in the waiting state, the amount of heat to be supplied to the element to be cooled 9 in the waiting state does almost not related to the heat radiating capability of the heat radiating unit, and consequently, a highly efficient heat radiating unit may be provided for maintaining the set temperature at which the element to be cooled 9 operates at a lower temperature.

In addition, according to Embodiment 1, an amount of heat may be supplied to the heater 40, which makes the temperature of the heat receiving portion 2 when the element to be cooled 9 is in the waiting state to become a temperature between a temperature Tr at which the element to be cooled 9 is in operation (53° C. in the example shown in FIG. 2) and an ambient temperature Te (20° C. in the example shown in FIG. 2). Preferably, an amount of heat may be supplied to the heater 40 which makes the temperature of the heat receiving portion 2 when the element to be cooled 9 is in the waiting state to become a median value of Tr and Te, that is, a temperature of (Tr+Te)/2(36.5° C. in the example shown in FIG. 2) or higher. As is seen from FIG. 2, by only supplying the amount of heat which makes the temperature of the heat receiving portion 2 when the element to be cooled 9 is in the waiting state to become (Tr+Te)/2 to the heater 40, if the element to be cooled 9 generates an amount of heat equal to or larger than several watts upon the element to be cooled 9 being activated, the temperature of the heat receiving portion 2 immediately reaches 50° C. or higher, which exhibits the advantage of the Embodiment 1. In addition, it is seen that when an amount of heat which is larger than the amount of heat described above, that is, an amount of heat which makes the temperature of the heat receiving portion 2 when the element to be cooled 9 is in the waiting state to become Te+0.8(Tr−Te) (a temperature lying 80% closer to the temperature at which the element to be cooled 9 operates between the ambient temperature and the temperature at which the element to be cooled operates) or higher, the element to be cooled 9 more preferably reaches faster the steady temperature at which it operates.

Here, in the case of the element to be cooled 9 being a semi-conductor laser device or laser diode (LD), by an electric current which is smaller than its oscillation threshold being made to flow to the LD, in place of heat being generated by the heater, heat can be made to be generated by the LD or the element to be cooled 9 itself. In the LD, when the electric current flowing therethrough is smaller than its oscillation threshold, no laser oscillation occurs, whereby no laser beam is outputted, that is, the laser oscillation is in an inoperable state. The power inputted into the LD in this state is lost, and therefore, heat is generated by the LD itself in accordance with the value of electric current which has flowed therethrough. In this configuration, the temperature of the LD can be maintained at almost a constant temperature both when the LD is not in operation (when no laser is oscillated or the LD is in the waiting state) and when the LD is in operation (when laser is oscillated) without using the heater 40. Consequently, since the LD can be made to operate in a constant oscillating state, that is, in an oscillating state which is free from variation in oscillation frequency or output, the advantage of the Embodiment 1 can particularly be exhibited.

As an application of the LD, there is raised a system in which an LD for emitting red and blue visible light beams is used as a light source of a video display unit. In the case of the video display unit, light from the light source is captured by the eyes of a human in the form of sensation as color or luminance. Since a variation in color, which is frequency, or luminance, which is output, is captured as a large variation in the sensation of the human, in particular, the stability of the light source is required in the video display unit. Because of this, in the system in which the LD is used as the light source of the video display unit, the advantage of Embodiment 1 is particularly important by which a variation in operation of the LD is suppressed when the video display unit is activated to operate. In addition, needless to say, the heater 40 may be provided separately even when the element to be cooled 9 is made up of the LD.

Here, while the configuration in which the temperature increasing heater 40 is not provided separately but the element to be cooled 9 generates heat by itself has been described by taking the LD as the example, the invention is not limited to the configuration so described. As long as the configuration is provided in which the element to be cooled 9 generates heat by itself by power inputted into the element to be cooled 9 which is sufficiently smaller than power required for the element to be cooled 9 to operate, the element to be cooled 9 may be made up of any types of elements or devices having an electric input which include other semiconductors than the LD or other electronic devices.

In addition, needless to say, the configuration in which the temperature increasing heater is not provided can also be applied to Embodiments 2 to 4, which will be described later.

The sealed container 1 is an airtight container which stores the liquid 6, vapor 7 and noncondensable gas 8 therein and may preferably be made of a metal which produces almost no chemical reaction between the liquid 6 and vapor 7 and the inner wall of the sealed container 1. For example, in the case of the liquid 6 being water, copper is preferably used as a material for the sealed container 1, and in the case of the liquid 6 being ammonia, it is recommendable to use a material such as aluminum or stainless steel which does not produce a noncondensable gas through a chemical reaction with the ammonia as a material for the sealed container 1.

The heat insulating portion 3 is a passage through which the liquid 6, vapor 7 and noncondensable gas 8 move. The heat insulating portion 3 may have its periphery exposed to a fluid such as air or brought into contact with a structure to radiate heat thereof. On the contrary, the heat insulating portion 3 may have a heat insulating material provided thereon to insulate itself against the loss of heat.

The heat radiating portion 4 has a function to cause the vapor 7 condensed to be liquefied and radiate latent heat emitted then to the heat sink 10. Projections may be provided on an inner surface of the heat radiating portion 4 so as to increase the heat conducting surface thereof for promotion of condensation of the vapor 7, and a passage for suctioning condensed liquid may be provided to make thin a condensed liquid film. In addition, fins may be provided on an outer circumferential surface of the heat radiating portion 4 so as to increase the heat conducting area for promotion of radiation of heat to the heat sink 10. It is noted that as has been described above, there may be a case where the gas-liquid interface 11 is positioned in the interiors of the heat insulating portion 3 and the heat radiating portion 4, and a part of the heat insulating portion 3 and the heat radiating portion 4 in which the gas-liquid interface 11 is so positioned plays a role of a passage or a container which accommodates the noncondensable gas 8 therein.

The noncondensable gas reservoir portion 5 has a function to accommodate the noncondensable gas 8 therein. There may be a case where the noncondensable gas reservoir portion 5 accommodates therein the liquid 6, vapor 7 and noncondensable gas 8 when the variable conductance heat pipe is not in operation. The noncondensable gas reservoir portion 5 is provided at an end portion of the variable conductance heat pipe which lies farthest from the heat receiving portion 2 with respect to the fluid passageway of the variable conductance heat pipe. Preferably, a configuration may be adopted in which the noncondensable gas reservoir portion 5 is provided at an uppermost portion of the constituent part of the variable conductance heat pipe, so that the liquid 6 that has flowed thereinto is allowed to flow into the heat radiating portion 4 downwards.

The liquid 6 is a liquid which can boil, evaporate and condense and may consist of a single-component fluid such as water and ammonia or a multi-component fluid such as an anti-freeze. The vapor 7 is a gas resulting from vaporization of the liquid 6 or part of the liquid 6. The noncondensable gas 8 is a gas which does not condense in the working environment where it is used, and under the normal environment, helium, argon, neon and nitrogen can configure the noncondensable gas 8. Preferably, the noncondensable gas is a gas which does not chemically react with the material of the sealed container 1, the liquid 6, and the vapor 7, and an inactive gas is preferably used. In addition, a non-condensable gas may be used which is generated by challengingly causing the sealed container 1 to react chemically with the liquid 6 in an initial stage of sealing the liquid, vapor and noncondensable gas into the sealed container 1.

Embodiment 2

FIG. 3 is a diagram showing the configuration of Embodiment 2 of the invention. While in Embodiment 1, the temperature of the heat receiving portion 2 can be set to an arbitrary temperature with less energy, when a main part 12 (of which the temperature needs to be controlled) in the element to be cooled 9 lies away from the heat receiving portion 2 (when a thermally interposed material 13 is interposed therebetween), even in Embodiment 1, there is generated a difference in temperature which corresponds to a thermal resistance of the thermally interposed material 13 is generated when the element to be cooled 9 is not in operation. Then, in Embodiment 2, by providing a heater 40 near a main part 12 in the element to be cooled 9, a difference in temperature which is generated by a heat input from the heater 40 and the thermal resistance of a thermally interposed material 13 can be compensated for, thereby making it possible to reduce a difference in temperature between when the element to be cooled 9 is in operation and not in operation.

In addition, in this case, since the difference in temperature which is generated by the input of heat from the heater 40 and the thermal resistance of the thermally interposed material 13 can be generated also when the element to be cooled 9 is in operation, the temperature of the main part 12 of the element to be cooled 9 can be controlled accurately by the input of heat into the heater 40.

In addition, by installing the heater 40 in the element to be cooled 9, the wiring is eliminated from the cooler (the variable conductance heat pipe 20), whereby the maintainability of the variable conductance heat pipe 20 is enhanced. Further, by providing a power supply and a control circuit which are provided outside the element to be cooled 9 within the element to be cooled 9, the maintainability is enhanced further. The configuration in which the heat 40 is installed in the interior of the element to be cooled 9 can, needless to say, be applied to the element to be cooled 9 which does not have the thermally interposed material 13 interposed between the heat receiving portion 2 and itself, that is, the element to be cooled 9 which is configured in the way described Embodiment 1.

Embodiment 3

FIG. 4 is a diagram showing the configuration of Embodiment 3 of the invention. In this configuration, a fan 14 is provided for forcing a cooling fluid to flow through a heat radiating portion 4 of a variable conductance heat pipe 20, an element to be cooled 9 is provided within a temperature sensor 15, and the fan 14 and the heater 40 are controlled with respect to outputs thereof by the temperature sensor 15 and a control circuit 16, so as to provide an optimum temperature control. The fan 14 may be a pump which causes the cooling fluid to flow to the heat radiating portion. Namely, the fan 14 may take any form, as long as it remains a cooler for cooling the heat radiating portion 4.

By adopting this configuration, the temperature of the element to be cooled 9 can be controlled through heating/cooling via the variable conductance heat pipe 20, whereby not only can a transition time to a set temperature and a waiting time of the user be shortened, but also the temperature at which the element to be cooled 9 can be controlled more accurately. Further, the outputs of the cooler such as the pump or the fan 14 and the heater 40 can be suppressed to minimum levels by the optimum control, thereby making it possible to implement the temperature control with conserved energy.

It is noted that while the case where the heater 40 is provided has been described above, as is described in Embodiment 1, the element to be cooled 9 may generate heat by itself without providing the heater 40. In this case, the element to be cooled 9 and the cooler may cooperatively perform output control.

Embodiment 4

FIG. 5 is a diagram showing the configuration of Embodiment 4 of the invention. There is provided a configuration in which the heat receiving portion 2 of the variable conductance heat pipe 20 and the element to be cooled 9 which includes a heater 40 are both covered by a heat insulating material 17. By adopting this configuration, not only can the temperature control efficiency of the element to be cooled 9 be enhanced, but also heat radiation while a set temperature is held is reduced so as to reduce the amount of consumed power that is required when the element to be cooled 9 is in a waiting state. To describe this in detail, while the property shown in FIG. 2 are those for the case where the element to be cooled and the heat receiving portion are not covered by an insulating material, when both the heat receiving portion 2 and the element to be cooled 9 which includes the heater are covered by the heat insulating material, in FIG. 2, property will be such that the portion where the properties of the variable conductance heat pipe rise is shifted leftwards. Consequently, an amount of heat which is required to make the temperature of the heat receiving portion when the element to be cooled 9 is in the waiting state to a temperature in a case where the heat receiving portion 2 and the element to be cooled 9 are not covered by the heat insulating material becomes less than an amount of heat in a case where the heat receiving portion and the element to be cooled are not covered by the heat insulating material.

It is noted that while the configuration has been described here in which both the heat receiving portion 2 and the element to be cooled 9 which includes the heater are both covered by the heat insulating material 17, even with a configuration in which either one of the heat receiving portion 2 and the element to be cooled 9 is covered by the heat insulating material, there is provided the advantage that compared with the configuration in which no heat insulating material is used, the amount of consumed power required when the element to be cooled is in the waiting state is reduced.

While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

1. A variable conductance heat pipe comprising a sealed container in which a working fluid and a noncondensable gas are sealed, the sealed container including a heat receiving portion to which an element to be cooled is provided, and heat radiating portion, wherein an amount of heat is supplied to the heat receiving portion when the element to be cooled is in a waiting state.
 2. The variable conductance heat pipe according to claim 1, wherein the amount of heat supplied to the heat receiving portion is an amount which makes a temperature of the heat receiving portion when the element to be cooled is in the waiting state to become a median value or higher, the median value being a median of a temperature Tr at which the element to be cooled operates and an ambient temperature Te.
 3. The variable conductance heat pipe according to claim 2, further comprising a heater which supplies the heat receiving portion or the element to be cooled with an amount of heat which makes the temperature of the heat receiving portion when the element to be cooled is in the waiting state to become the median value or higher.
 4. The variable conductance heat pipe according to claim 2, wherein when the element to be cooled is in the waiting state, the element to be cooled generates an amount of heat which makes the temperature of the heat receiving portion to become the median value or higher.
 5. The variable conductance heat pipe according to claim 3, further comprising a cooler provided to the heat radiating portion, wherein the heater and the cooler cooperatively perform output control.
 6. The variable conductance heat pipe according to claim 4, further comprising a cooler provided to the heat radiating portion, wherein the element to be cooled and the cooler cooperatively perform output control.
 7. The variable conductance heat pipe according to claim 1, wherein the element to be cooled is covered with a heat insulating material.
 8. The variable conductance heat pipe according to claim 3, wherein the heat receiving portion is covered with a heat insulating material.
 9. The variable conductance heat pipe according to claim 1, wherein the element to be cooled is a semi-conductor laser device. 